1. Field of the Invention
The invention relates to micro electromechanical systems, particularly to micro electromechanical switches and structures for testing the same. More specifically, the invention relates to test structures and test methods to acquire reliability and qualification data in order to characterize MEMS switch performance with statistical significance.
2. Description of Related Art
Micro Electromechanical Systems (MEMS) are being considered for possible switch structures in advanced high performance analog circuitry, in part, because of the improved switching characteristics over FET devices. For example, some MEMS-based RF switches are being developed with superior RF switching characteristics compared to other transistor-based switches, such as GaAs MESFETs, and the like.
While the development of these MEMS switches are in the early development stage, their performance must be empirically characterized; however, reliability and qualification methods for process enhancements and lifetime predictions are difficult to apply and require large sample sizes for accurate statistical determination.
In the qualification of MEMS relays, it is necessary to assess the overall performance of certain parameters including the degradation of performance over the life of the switch. These parameters will require quantitative measures with accompanying statistics in order to ascertain their longevity and reliability with statistical significance. Critical relays characteristics, such as activation and deactivation at certain activation/deactivation voltages, can be conveniently measured in a pass/fail fashion with the circuit design tolerance taken into account. These results are analyzed by plotting the cumulative fail in percentage versus lifetime under test in a lognormal scale. A statistical statement on the projected failure rate in normal operating lifetime can be obtained with an assigned level of confidence. In order meet higher and higher levels of reliability, statistical statements must be made with high precision and confidence. This means a larger amount of samples must be used in such test sequence.
Generally, the layout and fabrication of the MEMS devices makes the testing of large sample sizes impractical. For example, since each switch has at least four probe pads (two for the actuation and two for the contacts), an adequate sample size of switches would require either an extremely large number of I/O pads on the sample chip, or conversely, a large number of chips. These options quickly become expensive and impractical.